This invention relates to thermosetting casting compositions in which shrinkage is reduced by polymeric additives, while maintaining optical clarity.
It is well known that when monomers are polymerized, a volume reduction occurs. This shrinkage causes changes in dimensions, internal stresses in the cast article, cracking and/or separation from the mold wall, yielding surfaces of inadequate smoothness. The volume shrinkage can be reduced by two methods: either the monomer can be prepolymerized to low conversion before charging to the molds, or a `syrup` can be prepared by dissolving a previously prepared polymer in the monomer to be cast. Preparing the syrup is the preferred method since it is inherently simpler and economically more attractive. It is to this method that the present invention relates.
It is well known in the art that thermoplastic additives incorporated into unsaturated polyester resins, reduce shrinkage on curing of these resins. (Fritz M. Wright--New "No Shrink" Polyester Resin for BMC, SMC and Wet Moulding--Paper 12A, 27th Annual Conference, Proceedings--Reinforced Plastics/Composites Institute (1972). These additives function by phase separation from the resin matrix and the creation of microvoids. The resultant cured system is opaque. The present invention provides compositions in which shrinkage upon curing is reduced, but at the same time, optical clarity is maintained. This is important in the casting of monomers such as diethylene glycol bis (allyl carbonate) (ADC) which are used in optical and glazing applications.
Unsaturated thermoplastic materials have been prepared previously (U.S. Pat. No. 4,059,616). However, the anionic polymerization techniques used often imparted a slight discoloration to the polymer. Such colorations are unacceptable for use in the present invention. The thermoplastic additives used in the compositions of the present invention are prepared by free radical polymerization which yields completely colorless polymers. The above U.S. patent states that free radical polymerizations do not yield useful polymers.
U.S. Pat. No. 4,139,578 teaches the use of unsaturated polyesters, in conjunction with a second monomer as shrinkage reducing additives for ADC. It is an object of the present invention to bring about a reduction in shrinkage by the use of a copolymer alone.